light problem when combining cubes

I am making a dark-lighted maze, but when I add light to the scene, it shows the overlapping of the different walls/cubes by displaying white pixels. I already tried combining the cubes with an asset called ''Mastercube'', and while it did do the job of combining the meshes, it didn't fix the lighting problem.

Is there any other way I can fix this?

please keep in mind that I am merely a beginner with Unity (as you can probably see from my game)